10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Global Survey of Submicron Aerosol Acidity (pH)
BENJAMIN A. NAULT, Pedro Campuzano-Jost, Douglas Day, Weiwei Hu, Brett Palm, Jason Schroder, Roya Bahreini, Joost de Gouw, Huisheng Bian, Jack Kodros, Simon Clegg, John Crounse, Jack Dibb, Paul Wennberg, Felipe Lopez-Hilfiker, Eloise Marais, Ann M. Middlebrook, J. Andrew Neuman, John Nowak, Jeffrey R. Pierce, James Roberts, Joel A. Thornton, Patrick Veres, Jose-Luis Jimenez, et al., University of Colorado-Boulder
Abstract Number: 549 Working Group: Aerosol Chemistry
Abstract Aerosol acidity (H+, often expressed as “pH” defined in various ways) is an important property that influences uptake and partitioning of gases, and homogeneous and surface aqueous reactions of key inorganic and organic compounds. As there is currently no rapid method to directly measure ambient aerosol acidity, a thermodynamic model, constrained by both inorganic aerosol species (e.g., NH4, NO3, SO4, Cl) and at least one inorganic gas (HNO3, NH3, or HCl), is used to provide reliable estimates of aerosol acidity. In this study, we calculated submicron (smaller than 1 µm) aerosol pH from the NASA ATom, “pole-to-pole” series of aircraft campaigns that cover both the Pacific and Atlantic ocean basins in 4 different seasons. The E-AIM thermodynamic model was used with measurements by an Aerodyne high-resolution time-of-flight aerosol-mass-spectrometer (HR-ToF-AMS) of inorganic aerosol species, along with inorganic gas measurements from other mass spectrometers and ion chromatographs. We compare the results with those for the NASA KORUS-AQ, SEAC4RS, DC3, and ARCTAS aircraft campaigns, as well as several ground-based studies, and NOAA and NSF aircraft-based campaigns. We find urban submicron aerosols are typically highly acidic (pH ~ 1 – 2), although the least acidic compared to other environments. Rural/remote continental submicron aerosols are more acidic (pH ~ 0), and remote oceanic ones are the most acidic (pH ~ -1). We compare these results with results from multiple chemical transport models. We find that the aerosol acidity calculated from the measurements are typically 0.5 – 4 pH units lower than the chemical transport models for all locations around the world. This difference would result in dramatic differences in the calculated rates for some important chemical processes. The differences between the chemical transport models and the results calculated from the field observations are currently being investigated, including the impacts of sea salt emissions, meteorology, and atmospheric burden of ammonia and ammonium on aerosol acidity. These results inform the aerosol acidities that should be used in laboratory experiments of submicron aerosol chemistry. Additionally, we demonstrate the importance of accurate aerosol acidity in chemical transport models to ensure these models are properly representing these uptake, partitioning, and homogeneous and surface aqueous reactions of key inorganic and organic compounds.